Image: Michael Dietze/GFZ

On July 14, the Ahr River in Germany began to rise as the unprecedented rain continued to fall. It didn’t stop rising until nearly 200 people were dead, in flash floods that reached seven meters high and caused an estimated $10 billion in damages.

“The flood in the valleys of the Eifel was far more violent, faster and more unpredictable than we had previously assumed for such an event in the center of Europe,” says Michael Dietze of the Helmholtz Centre Potsdam – GFZ German Research Centre for Geosciences.

Dietze, a postdoctoral scientist, was in the southern Eifel region that day doing field work with colleagues. He had just recently placed seismic monitors at a site along the Ahr to monitor ground and sediment motion patterns when floods occur. Dietze knew immediately that the July 14 event was no “normal” flood and, as it turned out, his monitoring equipment was long gone with the raging river too.

One thing that became clear to him is that in a world of changing climate, the models to predict such flooding may need to be reworked. That’s because they’re not taking into account some of the real-world conditions.

Dietze and colleague Ugur Ozturk discussed their observations and concerns in a letter published Friday in the journal Science. They noted that some causes for the flooding are seen more often in the desert, or in tropical regions, but not in Central Europe. High on their list was the way that dead trees, debris and river sediments contributed to the catastrophe by causing “dams” on the river and forcing the water behind them higher.

There were other factors besides the deluge and the damming too. The rain fell on already saturated soils and began to run off the valley slopes much faster than anticipated—about 100 times faster, Dietze and Ozturk said.

“The water developed enormous erosive power,” said the authors. “On the one hand, it dug channels into the slopes and was able to flow even faster (in them). On the other hand, it mobilized considerable amounts of sediment and dead wood. Once in the main valleys, the tree trunks and branches drifted towards bridges.”

Other concerns included impacts to roads and houses that collapsed in the town of Blessem, as well as the stability of the earthen Steinbach Dam about 35 kilometers upstream from the flooding. The flooded dam eroded, and ultimately a feared breach never happened, but the threat to even more villages below the dam put a spotlight on how interconnected the climate impacts are.

These types of events are likely as climate change continues to heat the planet and episodes of extreme rainfall become more frequent. That’s why these other “coupled” factors, like debris flows, need to be considered.

So GFZ, in partnership with the NatRiskChange Research Training Group at the University of Potsdam, is launching a new research project to map all of the impacts of Germany’s July 2021 flooding disaster. They’re using airborne laser scans to produce 3D models of how the landscape has changed when compared with the pre-flood scene, so that they can integrate the data into better climate models.

“Research must now begin to understand precipitation-induced floods not only as a phenomenon of too much fast-flowing water,” says Dietze. “We also have to include the associated self-reinforcing effects, some of which are also favored by climate change.”

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